1. Field of the Invention
The present invention relates to a switch structure having a pair of main contacts for making a required electric connection between selected parts in an associated electric circuit, and particularly to such a switch structure having means responsive to one or the other main contact being partly melted and stuck to the counter contact for forcedly breaking and opening the so closed circuit.
2. Description of Related Art
Referring to FIG. 13, a conventional switch 10 is used in an electric powered carpenter tool for starting or stopping its dc motor M. The switch 10 can selectively make a series-connection of the motor M to an electric power supply E, thus putting the motor M in running condition. Otherwise, the switch 10 can selectively short-circuit the opposite terminals of the motor M to effect regenerative braking.
Referring to FIG. 14, the switch 10 has a switching mechanism installed in its casing 11. The switching mechanism can be operated by an operating lever 12. It comprises a seesaw-like lever 14 balanced in the middle with the aid of a triangular fulcrum 13, and the lever 12 has two movable contact elements 17 and 18 fixed to its opposite ends. The seesaw-like lever 14 can be tilted clockwise or counterclockwise by moving the operating lever 12 back and forth so that one end goes up when the other end goes down, allowing these movable contact elements 17 and 18 to be alternately put in contact with the counter stationary contact or 20. Two opposite terminals of the switch, which are hereinafter referred to as "Main Terminal B" and "Brake Terminal C", are connected to the opposite stationary contact elements 19 and 20, and the middle terminal, which is hereinafter referred to as "Common Terminal A", is connected to the triangular fulcrum 13 of the seesaw-like lever 12. The common terminal A is connected to one terminal of the motor M. The main terminal B is connected to one terminal of a dc power supply E, and the brake terminal C is connected both to the other terminal of the motor M and to the other terminal of the dc power supply E.
As shown in the drawing, the operating lever 12 has a spring-biased slidable piece 15 fixed to its elongated arm 16. Specifically the elongated arm 16 has a spring 15a contained in its cavity to urge the slidable rounded projection 15 against the seesaw-like lever 14 all the time. When the operating lever 12 is moved back and forth, the slidable rounded projection 15 is moved back and forth on the seesaw-like lever 14 to put the first or second movable contact element 17 or 18 in contact with the first or second stationary contact element 19 or 20. As seen from FIG. 14, a sub-switching device for supplying the motor M with electricity is made up by the seesaw-like lever 14, the second movable contact element 18, the second stationary contact element 20, the main terminal B and the common terminal A. Another sub-switching device for braking the motor M is made up by the seesaw-like lever 14, the first movable contact element 17, the first stationary contact element 19, the brake terminal C and the common terminal A.
When the running motor M is made to stop, the operating lever 12 is released to allow the slidable piece 15 to move leftward (in the direction indicated by arrow P2) under the influence of coiled spring 61. Then, the second movable contact element 18 departs from the second stationary contact element 20 as indicated by arrow P3, and at the same time, the first movable contact element 17 is put in contact with the first stationary contact element 19 as indicated by arrow P4, thus short-circuiting the motor M to cause the regenerative braking on the motor M. When it is desired that the motor M is put in running condition, the switch is operated to work in the opposite way.
When the sub-switching device is used repeatedly for supplying the motor M with electricity, the second movable and stationary contact elements 18 and 20 are liable to be worn and roughened on their surfaces with the result that these contact elements are melted and stuck together. When this happens actually, the motor M is continuously supplied with electricity even though the operating lever 12 is released.
It is most likely that an electric carpenter tool equipped with such switching mechanism goes to destruction when its life is close to end. To prevent the motor M from running in the destructive mode it has been proposed to equip the switching mechanism with forcedly circuit-opening means for emergency use. This is required particularly in electric saws, grinders and other handhold type of electric-powered carpenter tools for safety.
An example of a switch equipped with forcedly circuit-opening means is shown in Japanese Patent 62-115617(A). As seen from FIG. 15, it comprises a main switching mechanism for opening or closing a motor-and-power supply series-connection, and an auxiliary switching mechanism series-connected to the main switching mechanism.
As seen from the drawing, the switch comprises a plunger type of operating knob 30, a position retainer 31 fitted on the bottom of the plunger knob 30, a resilient support 32 abutting on the lower side of the position retainer 31, a main movable contact element 33 ganged with the resilient support 32, and a main stationary contact element 34 confronting the main movable contact element 33.
The auxiliary switching mechanism is made up by auxiliary movable and stationary contact elements 35 and 36. As the position retainer 31 pushes the auxiliary movable contact element 35 against the auxiliary stationary contact element 36, the auxiliary movable contact element 35 is applied to the stationary contact element 36 all the time.
In operation the pushing of the operating knob 30 causes the main movable contact element 33 to be put in contact with the main stationary contact element 34 through the agency of the position retainer 31 and resilient support 32, and the releasing of the operating knob 30 causes the main movable contact element 33 to depart from the main stationary contact element 34 under the resilient influence of the resilient support 32, while the position retainer 31 is allowed to rise and follow the operating knob 30. Thus, the auxiliary movable contact element 35 is retained to be put in contact with the auxiliary stationary contact element 36 all the time by allowing the position retainer 31 to slide on the auxiliary movable contact element 35.
Assume that the main movable contact element 33 is melted and stuck to the main stationary contact element 34 when the main switching mechanism turns on, as shown in FIG. 16. As the operating knob 30 is spring-biased upward, it is raised, leaving the position retainer 31 below, and the main movable contact element 33 is stuck to the main stationary contact element 34, thus remaining in the "on" condition. Then, the position retainer 31 is pushed rightward by the auxiliary movable contact element 35, departing from the auxiliary stationary contact element 36 to prevent the motor M from being supplied with electricity from the power supply E.
As no force is applied to the position retainer 31 from the stuck main movable contact element 33, the auxiliary movable contact element 35 is allowed to depart from the auxiliary stationary contact element 36, thereby forcedly breaking the series-connection between the motor M and the power supply E.
The forcedly circuit-opening switch, however, is large in size. The number of parts to be assembled and hence, the assembling and manufacturing cost increase accordingly. Also, disadvantageously the inner resistance of the switch is increased. The main and auxiliary switching parts must be connected outside of the switch housing. This adds one extra step to the assembling work. The contact structure is designed for microswitch, thus limiting the quantity of electric current. The switch, therefore, cannot be used as power switch in an electric-powered carpenter tool such as an electric saw; a relatively heavy current flows in the electric-powered carpenter tool. Also disadvantageously, the switch cannot be equipped with brake contacts.